Preheat oven to 180°C (350°F) and grease and line a 18x28 cm (7x11 inch) slice tin.
In a large bowl, mix all the dry ingredients until well combined.
Add the butter and vanilla, and mix until well combined.
Tip the mixture into the prepared tin, spread out evenly and press firmly.
Bake for 15–20 minutes until golden.
Cool in the tin, then ice (optional) and cut into squares.
